Testing of NPT and NPTF Connections
NPT and NPTF connections are commonly used in the water jet cleaning industry. As a higher and higher working pressure is being used in this industry, a concern has been raised about the use of NPT and NPTF connections. Lees and Crofton* did some testing in an attempt to address this issue.
Testing was done with two popular sizes (1/4” and ½”) of NPT and NPTF connectors (with NPT and NPTF threads mixed and matched). A “Threadmate” (a Parker Hannifin, Inc. product) sealant, PTFE tape, and dry condition were used as another testing variable. Two methods of applying torque were used. In the first method, the tightening torque was progressively increased after each leak. In the second method, the maximum torque (100 ft-lb for ¼” and 150 ft-lb for ½” connections) was applied once and once only. The fittings for the tests were made from cold drawn 316 stainless steel bar. Test fluid was hydraulic oil with a very low viscosity. Connections in test were immersed in water to check leakage of oil. Testing pressure was limited to 80,000 psi maximum.
Even though NPTF threads (also known as “Dryseal”) are designed to form a better seal (because no clearance is allowed between the rest and root of the thread), the test results showed that no one combination (among NPT/NPT, NPTF/NPTF, NPT/NPTF) consistently outperformed the others.
Under dry condition (no sealant or tape), the maximum torque tightening method had a higher leakage pressure than the progressive tightening method probably because of plastic deformation of threads. With the PTFE tape and the Threadmate sealant, the leakage pressure was significantly higher than without (dry condition) and the progressive tightening method performed as well as (if not better) the maximum torque method. Therefore the fittings can be reused if they are recoated with PTFE tape or sealant. There was no consistent difference between the PTFE tape and the Threadmate sealant in terms of leakage pressure. For ¼” thread it appears that there was an optimum torque of around 50 ft-lb, beyond which the leakage pressure got lower. There were evidences, when a proper tightening torque and a proper sealant were applied, both the ¼” and ½” threads were capable of withstanding 80,000 psi without blowout or even leakage (a good safety margin for an operating pressure of 20,000 psi).
Authors’ disclaimer: These results and comments are not intended to be recommendations or approvals for the use of any type of fitting at any pressure.
William, A. and Crofton, P. Shaun (1999) Factors influencing the leakage
characteristics of NPT and NPTF threaded connectors, Proceedings of the 10th
American Waterjet Conference,
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